Use system locale when Accept-Language header is not provided

Remove en_US as the default language when no header is provided, and use
None instead. Upon translation None will be defaulted to system as it
was before the translation changes.

Fixes bug: #1214476

Change-Id: I0fe22c526710e69ae0731e7d0b42170e6f3a8523
Deal with 501 errors from NVP correctly

This is done by adding the error code to the exception
handlers dictionary as done for other error codes.

Fixes bug #1215583

Change-Id: Ic876c6bccaf94cae626dfc908c4a34dfbcead406
Separate l3 db tests for l3 agent into their own test case

Bug 1215871

This patch does a simple refactoring of test_l3_plugin, pushing out tests
aimed at validating the interactions of the l3 agent with the server.

These tests explicitly use TestL3NatPlugin, whereas all the
other tests use a configurable plugin, which might be specificed by a child
class. This might lead to confusion and possibly errors in unit tests for
child classes, especially those not using the l3 agent - for which running
these test is also superfluous.

Change-Id: Ia9ed320ea775fc548e6a5b711c67c9f3b0ae2f6d
Imported Translations from Transifex

Change-Id: I9300e32b84f02492c8cf32cfff65f3eb207672d5
Merge "Separate l3 db tests for l3 agent into their own test case"
Merge "Use system locale when Accept-Language header is not provided"
Merge "Deal with 501 errors from NVP correctly"